The present invention relates generally to eddy current test coil structures, and particularly to a pole and coil assembly adapted to concentrate magnetic lines of force in a limited area of the coil to thereby concentrate the resulting eddy currents produced in a metal product by the concentrated lines of force as the metal product is directed through the coil.
Heretofore, in the art of continuously examining elongated metal product directed past an examining station, the practice has usually been to direct the product through a circular coil or coils designed to induce eddy currents in the product in a uniform manner. The magnetic lines of force generated by the coil or coils enter the product in a generally uniform manner so that the product, or at least the periphery thereof, is uniformly examined.
Such a test coil is shown schematically in U.S. Pat. No. 3,735,084, issued on May 22, l973, in the name of John M. Urbanic et al. The coil is employed to receive and pass therethrough a tubular product formed from a strip of metal welded along aligned abutting edges thereof. The coil and associated circuitry in this patent is adapted to examine the wall and the seam weld of the product and to reject any portions thereof having a defect. It has been found, with the use of such coils, that tubing having a structurally sound wall and seam weld has been rejected on the basis of dents, scratches or other surface discontinuities on the wall thereby generating, unnecessarily, substantial amounts of a scrap material. On the other hand, at the same time, such a search coil will function to accept and pass, as good tubing, tubing lengths having extremely small discontinuities or "microleaks" in the seam weld, which lengths should have been rejected. It can be appreciated that if the welded tube is employed in a heat exchange unit, in which the tube contains and directs fluids therethrough under pressure in the heat exchanging process, any minute crack or discontinuity in the tube weld will disable the complete unit.
Another approach of the art to examining traveling, elongated metal product is the use of eddy current probes, such as shown in the Lorenzi et al. U.S. Pat. No. 3,495,166. Such probes tend to concentrate the eddy currents inducing magnetic field to such an extent that defects and discontinuities lying outside the concentrated field are missed. In the Lorenzi et al patent, for example, the probe is designed to search for defects having a dimension that is substantial in one direction only.
Thus, the problem of adequately examining the longitudinal weld of seam welded product for even the minutest discontinuity, without the attendant result of rejecting otherwise good and sound tubing, has not been solved by the type of test coil that simply surrounds the tubing and uniformly directs magnetic lines of force into the tubing wall, or by the probe that examines a very restricted area of a product or searches for a particular type of defect only.
Other U.S. patents showing probe and/or product encircling test coils for examining elongated product are Knerr et al. No. 2,124,579, Paivinen No. 2,744,233, Datt et al., No. 2,980,848, Hochschild No. 3,056,081, Quittner No. 3,273,055, Brown No. 3,395,339 and Puidak No. 3,449,661. In the Datt et al. patent, a pair of excitation and sensing coils are shown separated by a partition of "Mu Metal". The Mu Metal, with the coils, encircles the workpiece to be examined so that a magnetic field is generated and directed uniformly into the entire periphery of a workpiece to be tested.
FIG. 12 of the above Knerr et al patent shows a device for sensing flaws in tubular product using two spaced apart primary excitation windings completely surrounding the product, and secondary sensing windings located intermediate the excitation windings. The excitation windings generally uniformly produce eddy currents in the product, while the sensing windings detect flaws in the product only in the area of the sensing windings. In addition, the detection windings are shown shielded from the excitation windings by laminations of iron, copper or the like.
In addition, Flaherty et al. U.S. Pat. No. 3,430,134 discloses the use of three coils to track and test seam welds in pipe, the coils being magnetically isolated from each other to increase sensitivity to the weld location. Further, the coils are elongated in the direction of the weld so that an integrating and averaging effect is provided, which effect reduces sensitivity of the probe to minor discontinuities in the weld.